require yaml where its used, instead of in gemspec.

adapters/database_cleaner-active_record/lib/database_cleaner/active_record/base.rb

@@ -1,6 +1,7 @@
 require 'active_record'
 require 'database_cleaner/generic/base'
 require 'erb'
+require 'yaml'
 
 module DatabaseCleaner
   module ActiveRecord

adapters/database_cleaner-ohm/spec/database_cleaner/ohm/truncation_spec.rb

@@ -1,5 +1,6 @@
 require 'ohm'
 require 'database_cleaner/ohm/truncation'
+require 'yaml'
 
 module OhmTests
   class Widget < ::Ohm::Model

adapters/database_cleaner-redis/spec/database_cleaner/redis/truncation_spec.rb

@@ -1,5 +1,6 @@
 require 'redis'
 require 'database_cleaner/redis/truncation'
+require 'yaml'
 
 RSpec.describe DatabaseCleaner::Redis::Truncation do
   around do |example|